A type of tunnel liner that has been used abroad for about 50 years and has recently been introduced into the United States comprises modular precast concrete elements that are assembled in a tunnel as digging progresses, to build up a liner behind the digging operation and more or less in step with it. Each such modular element comprises a ring segment having arcuate, substantially concentric inner and outer faces, opposite end faces, and opposite side faces that are adapted to abut side faces of similar, circumferentially adjacent elements. A certain number of such arcuate elements are assembled with one another to form a ring, starting at the bottom of the ring and progressing up along both sides of it. The arcuate elements are usually so dimensioned that there is a small gap between the two uppermost elements in the ring, which gap is filled by a key block that is inserted to complete the ring.
As each new arcuate element is added to a ring under construction, it is locked to an already placed circumferentially adjacent element. For such connection of the elements that comprise a ring, each arcuate element has steel loops at each side of it, each projecting a little beyond its adjacent side face of the element to overlap similar loops on a circumferentially adjacent element. A tapering wedge pin driven into the overlapped loops locks the elements to one another. The loops are so oriented that the wedge pins can be driven into them from inside the ring, in the radially outward direction.
Each of the loops on an arcuate modular element of this prior tunnel liner is located in a pocket or bay that opens to the arcuate inner face of the element and to the side face beyond which the loop projects. Each such pocket receives part of a cooperating loop or loops on a circumferentially adjacent element, to enable the loops to overlap. Because of the configuration of the pockets in relation to the loops, each modular element must be moved in a substantially circumferential direction in order to bring it into assembled relationship to the circumferentially adjacent element to which it is connected. The gap between the uppermost arcuate elements of a ring--subsequently filled by the key block--accommodates such circumferential motion of the last arcuate element assembled into the ring.
After each ring is assembled, the next ring forwardly along the length of the tunnel is assembled in a similar manner. The arcuate elements of each new ring are usually placed in circumferentially offset relation to those of the last-finished ring so that the arcuate elements of successive rings along the tunnel are staggered like bricks in a wall.
The arcuate elements of the ring being asembled are established in their properly staggered positions relative to the elements of the previously assembled ring by means of dowel rods that are received in wells in the completed ring and project forwardly from it. However, because each modular element must be moved circumferentially--not axially--in being assembled into a ring, the dowel rods must be inserted into the finished ring one at a time, each being installed just before a new element of the next ring is set in place. For cooperation with the dowel rods, each element has an axially extending dowel rod groove in each of its end surfaces. When an element is brought to its proper assembled position, its dowel rod groove receives the rod that positions the element.
Although the dowel rod grooves in a pair of circumferentially adjacent modular elements cooperate to define a well in which a dowel rod is received that positions the next axially adjacent element, the dowel rod performs no further function after an element has arrived at the position that the dowel rod defines; that is, the dowel rod does not lock the elements of one ring to those of the rearwardly adjacent ring, any more than it locks circumferentially adjacent elements to one another. For ring-to-ring securement, each element has a pair of bores extending through it in the axial direction, and long bolts are inserted through these bores and threaded into sockets in the elements of the rearwardly adjacent, previously assembled ring.
Typically an arcuate tunnel liner element is three feet wide, as measured along the length of the tunnel, and therefore each of the bolts that secures it to rearwardly adjacent elements must be a little more than three feet long. Often a certain amount of manipulation of a bolt is necessary to get it properly engaged in the threaded socket that is to receive it, and then additional time has to be consumed in turning the bolt into the socket. Inserting and fastening each such bolt can take from about half a minute to a full minute, and sometimes longer. In tunnel work, time is very expensive, being typically estimated at more than $300 per working hour. With as many as six or eight arcuate elements in a typical ring--and substantially more in a large diameter tunnel--and with, typically, two bolts per element, the cumulative time required for inserting and screwing in the bolts represents a very substantial item of cost. Although substantially less time is consumed in inserting the dowel pins that position the elements, the number of such insertions that has to be performed in the assembly of a complete tunnel liner entails something more than a negligible cost.
During their history of about half a century, modular tunnel liners of the above described type have been improved in certain respects. See for example British Pat. No. 2,004,931. Heretofore, however, the skilled artisans working in this field have failed to devise an expedient that would eliminate or avoid the costs and inconveniences of dowel pins and long bolts.